Hitherto, as a biaxial tensile testing machine, techniques disclosed in PTL 1 and NPL 1 are widely known. In the biaxial tensile testing machines disclosed in the Patent Literature, four gripping jigs gripping a thin sheet test piece are respectively fixed to tensile test shafts of two hydraulic cylinders disposed on each of two axes perpendicular to each other in order to stretch the cross-shaped thin sheet test piece in four directions. According to the biaxial tensile testing machine, when the four hydraulic cylinders are driven, the thin sheet test piece gripped by the gripping jigs is stretched in four directions, whereby a biaxial tensile test is performed on the thin sheet test piece.